ZULU, I don't disagree with your basis of your assertion but I think you were actually referring to "mainline" and not "mainstream" denominations. Please allow me to make the important distinction...
"Mainline" denotes a clear historcal characteristic while "mainstream" implies a subjective political or theological meaning. The "mainline" denominations are those Protestant denominations from which the newer evangelical denominations branched off during the last century. The PCUSA is a mainline denomination, but the conservative PCA, OCP and ECP denominations which split off from it are not mainline. The American Baptists and National Baptists are mainline, while the conservative Southern Baptists who separated from them are not mainline. And so it goes with the other once-vibrant hollow shells of mainline unbelief.
Interestingly, members of the newer and more theologically conservative evangelical denominations now outnumber mainline members, and evangelical denominations continue to grow while all of the mainline denominations continue to lose members. If either group can legitimately claim to be "mainstream" today, it would have to be the evangelical churches.
You are correct. My terminology was wrong.
The bright spot is the evangelicals. They need to infiltrate and take over all the existing mainline Christian sects.